Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of fiber optic networks. In a WDM system, plural optical signals or channels are carried over a single optical fiber with each channel being assigned a particular wavelength. Such systems typically include a demultiplexer for separating and supplying the optical channels to corresponding optical receivers.
In order to increase system capacity further, data rates associated with each optical signal have increased. In particular, data rates have increased from 2.5 Gbits/second to 10 Gbits/second in commercially available systems, and 40 Gbits/second per channel systems are currently in development.
At relatively high data rates, such as 40 Gbits/second, different modulation formats have been considered for providing optimal transmission. One such format, known as Carrier Suppressed Return-To-Zero (CS-RZ), has been proposed for transmitting 40 Gib/second optical signals. In accordance with the CS-RZ format, the carrier or central wavelength of an optical signal spectrum is suppressed, while optical signal sidebands in the spectrum carry the transmitted data. CS-RZ optical signals have the advantage that they can be transmitted into fiber spans with a higher channel power than other conventional modulation formats before degradation occurs due to fiber nonlinear effects. See Miyamoto et al., Electronics Letters, vol. 35, no. 23, Nov. 11, 1999, pp. 2041-2042, incorporated by reference herein.
40 Gbit/second CS-RZ signals, however, are susceptible to chromatic dispersion, a non-linear effect in which spectral components of an optical signal propagate through the optical fiber at different speeds with higher frequency components traveling slower than lower frequency components. Chromatic dispersion can result in spectral components of one pulse arriving at a receiver at substantially the same time as a succeeding pulse, thereby causing degraded receiver sensitivity and a relatively high bit error rate.